I am just getting started with WP7 development, MVVM, etc. Your toolkit (v4 preview) looks very promising. I too would like to see a standard implementation of INotifyPropertyChanging for the same reason
the previous poster mentioned. It seems to fit well on your ObservableObject class. I created my own ObservableObject class and derived from yours, but the need to raise the NotifyPropertyChanging and NotifyPropertyChanged events immediately before
and after the property set meant that I had to re-implement all of the meaningful methods within my subclass. Are you still planning to add this in v4? If so, do you have an ETA?

Yes it is still planned. I am fighting with a memory leak in the Messenger in the moment, so it is hard to set a date on a release, but rest assured I am working very actively on MVVM Light at the moment.